Mammary glands of young female BALB/c mice develop hyperplastic nodule-like alveolar lesions (NLAL) in response to a 24-h exposure to 7,12-dimethylbenz(a)anthracene (DMBA; 2 micrograms/ml) on Day 3 during a 24-day organ culture. Experiments were conducted to determine if 12-O-tetradecanoylphorbol-13-acetate (TPA), a known tumor promoter, can influence the development of DMBA-induced NLAL in mammary gland organ culture. Mammary glands were incubated with TPA (25 ng/ml) for various periods of time in the presence of appropriate hormone combinations. Results indicated that the presence of TPA in the medium between Days 9 to 14 of the culture period enhanced both the incidence and multiplicity of DMBA-induced NLAL; however, it was ineffective when included in the medium between Days 4 to 9 or 19 to 24 of the organ culture. Toxicity of TPA was evident when it was present during the entire culture period subsequent to DMBA treatment. Computer-assisted image analysis of NLAL in the glands determined the area covered by these lesions within the gland. It was observed that 7.8% of the area was covered by NLAL in DMBA plus TPA-treated glands as compared to 2.5% by DMBA treatment alone. These results provide a model for initiation-promotion studies of mammary carcinogenesis in vitro, as well as a modified approach for quantitative analysis of structural alterations.